The present invention relates to fractionation of gas mixtures by selective adsorption and is particularly concerned with recovery from mixed gas streams, such as atmospheric air, of an oxygen-enriched gas fraction with or without accompanying recovery of a high purity nitrogen product.
Systems for separation of air components by selective sorption to recover oxygen and/or nitrogen are well known in the art. Typically, most of the known systems employ a zeolite molecular sieve adsorbent bed for selective retention of nitrogen from the feed gas while collecting an oxygen-enriched product stream as primary effluent. Periodically, as the sorbent bed reaches a designed level of sorbed gas, the bed is desorbed and/or purged to remove contained nitrogen, before being returned to on-stream operation. The adsorption-desorption cycles rely chiefly on swing in pressure level of these steps in the operating cycle. Also, to maintain continuity of operation, a number of such adsorbent beds are operated in parallel, so that while one bed is operating on the adsorption step of the cycle, one or more companion beds are at various stages of regeneration.
In certain known systems, the feed gas charge is introduced into the adsorption column at an initial relatively high pressure during the adsorption step, then the pressure of the column is lowered to near atmospheric pressure or to a level below atmospheric to effect desorption of the column. The desorbed bed is purged, usually with part of the unadsorbed effluent from the adsorption step, followed by repressuring the bed to its initial high pressure level for repetition of the cycle. In certain systems applied to air fractionation, the desorption of the nitrogenaloaded adsorbent is effected under vacuum.
Air fractionation by vacuum swing adsorption (VSA) is described, for example, in U.S. Pat. Nos. 3,533,221, 3,957,463, 4,013,429 and 4,264,340. As described in the '429 patent, the operation involves a sequence of four principal steps: wherein (1) ambient air, freed of moisture and CO.sub.2 in a pretreatment column, is passed through a column in series therewith containing a bed of adsorbent selective for retention of nitrogen in the feed, while unadsorbed oxygen-enriched product gas is being recovered as primary effluent collected and stored in a surge chamber such as an expansible receiving vessel. The ambient air feed is continued until incipient breakthrough of air occurs or somewhat short of it, depending upon the desired concentration of the oxygen-enriched product. The flow of air to that bed is then discontinued and switched to a companion bed which had been previously regenerated and brought to the designed on-stream pressure level. The off-stream adsorbent bed is now (2) rinsed with high purity nitrogen collected from a previous step in the cycle, thus saturating the bed with nitrogen. The nitrogen saturated bed is next (3) evacuated thereby desorbing the high purity nitrogen, part of which is used as rinse gas (in step 2) and the remainder collected as desired high purity nitrogen product and stored in a surge chamber. After the evacuation of the bed to the desired level, (4) a part of the previously collected oxygen-rich product gas is withdrawn from the surge chamber and introduced into the bed to saturate the bed and to bring it back to the desired pressure level for initiating resumption of the ambient air feed and repetition of the cycle.
A similar operating sequence is disclosed in the '340 patent, except that the system is combined with an operatively associated thermal swing section for drying the wet nitrogen stream recovered by vacuum desorption of the pretreater column and the main nitrogen adsorption column.
In the operation of the prior systems, as described in the '429 and '340 patents, the time period duration for each of the steps of the cycle sequence is fixed by a preset program, the opening and closing of the various flow-control valves involved being under control of a cycle timer device.
In a copending patent application, Ser. No. 416,433, filed Sept. 9, 1982, now U.S. Pat. No. 4,472,177, a programmed system is disclosed wherein the time duration of the adsorption step and the nitrogen rinse step in a VSA process for air fractionation, is each automatically controlled by sensing the analyzed percent composition of the respective withdrawn product streams during these steps, and terminating the adsorption and rinsing, respectively, when preset composition levels are attained. Also, the extent of evacuation of the main adsorbent bed and of the pretreatment bed is controlled automatically to terminate when preset vacuum levels are sensed. In similar manner, the control system operates to terminate the repressuring step when a preset pressure level is attained. The flow rate of input atmospheric air remains contant.